How van der Waals interactions determine the unique properties of water

T Morawietz and A Singraber and C Dellago and J Behler, PROCEEDINGS OF
THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 113,
8368-8373 (2016).

DOI: 10.1073/pnas.1602375113

Whereas the interactions between water molecules are dominated by
strongly directional hydrogen bonds (HBs), it was recently proposed that
relatively weak, isotropic van der Waals (vdW) forces are essential for
understanding the properties of liquid water and ice. This insight was
derived from ab initio computer simulations, which provide an unbiased
description of water at the atomic level and yield information on the
underlying molecular forces. However, the high computational cost of
such simulations prevents the systematic investigation of the influence
of vdW forces on the thermodynamic anomalies of water. Here, we develop
efficient ab initio-quality neural network potentials and use them to
demonstrate that vdW interactions are crucial for the formation of
water's density maximum and its negative volume of melting. Both
phenomena can be explained by the flexibility of the HB network, which
is the result of a delicate balance of weak vdW forces, causing, e.g., a
pronounced expansion of the second solvation shell upon cooling that
induces the density maximum.